Without limiting the scope of the present invention, its background is described with reference to using optical fibers for communication and sensing in a subterranean wellbore environment, as an example.
It is well known in the subterranean well completion and production arts that downhole sensors can be used to monitor a variety of parameters in the wellbore environment. For example, during a treatment operation, it may be desirable to monitor a variety of properties of the treatment fluid such as viscosity, temperature, pressure, velocity, specific gravity, conductivity, fluid composition and the like. Transmission of this information to the surface in real-time or near real-time allows the operators to modify or optimize such treatment operations to improve the completion process. One way to transmit this information to the surface is through the use of an energy conductor which may take the form of one or more optical fibers.
In addition or as an alternative to operating as an energy conductor, an optical fiber may serve as a sensor. It has been found that an optical fiber may be used to obtain distributed measurements representing a parameter along the entire length of the fiber. Specifically, optical fibers have been used for distributed downhole temperature sensing, which provides a more complete temperature profile as compared to discrete temperature sensors. In operation, once an optical fiber is installed in the well, a pulse of laser light is sent along the fiber. As the light travels down the fiber, portions of the light are backscattered to the surface due to the optical properties of the fiber. The backscattered light has a slightly shifted frequency such that it provides information that is used to determine the temperature at the point in the fiber where the backscatter originated. In addition, as the speed of light is constant, the distance from the surface to the point where the backscatter originated can also be determined. In this manner, continuous monitoring of the backscattered light will provide temperature profile information for the entire length of the fiber.
Use of an optical fiber for distributed downhole temperature sensing may be highly beneficial during the completion process. For example, in a stimulation operation, a temperature profile may be obtained to determine where the injected fluid entered formations or zones intersected by the wellbore. This information is useful in evaluating the effectiveness of the stimulation operation and in planning future stimulation operations. Likewise, use of an optical fiber for distributed downhole temperature sensing may be highly beneficial during production operations. For example, during a production operation a distributed temperature profile may be used in determining the location of water or gas influx along the sand control screens.
In a typical completion operation, a lower portion of the completion string including various tools such as sand control screens, fluid flow control devices, wellbore isolation devices and the like is permanently installed in the wellbore. As discussed above, the lower portion of the completion string may include various sensors, particularly, a lower portion of the optical fiber. After the completion process is finished, an upper portion of the completions string which includes the upper portion of the optical fiber is separated from the lower portion of the completion string and retrieved to the surface. This operation cuts off communication between the lower portion of the optical fiber and the surface. Accordingly, if information from the production zones is to be transmitted to the surface during production operations, a connection to the lower portion of the optical fiber must be reestablished when the production tubing string is installed.
It has been found, however, that wet mating optical fibers in a downhole environment is very difficult. This difficulty is due in part to the lack of precision in the axially movement of the production tubing string relative to the previously installed completion string. Specifically, the production tubing string is installed in the wellbore by lowering the block at the surface, which is thousands of feet away from the downhole landing location. In addition, neither the distance the block is moved nor the speed at which the block is moved at the surface directly translates to the movement characteristics at the downhole end of the production tubing string due to static and dynamic frictional forces, gravitational forces, fluid pressure forces and the like. The lack of correlation between block movement and the movement of the lower end of the production tubing string is particularly acute in slanted, deviated and horizontal wells. This lack in precision in both the distance and the speed at which the lower end of the production tubing string moves has limited the ability to wet mate optical fibers downhole as the wet mating process requires relatively high precision to sufficiently align the fibers to achieve the required optical transmissivity at the location of the connection.
Therefore, a need has arisen for an apparatus and method for wet connecting optical fibers in a subterranean wellbore environment. A need has also arisen for such an apparatus and method for wet connecting optical fibers that is operable to overcome the lack of precision in the axial movement of downhole pipe strings relative to one another. Further, a need has arisen for such an apparatus and method for wet connecting optical fibers that is operable to overcome the lack of precision in the speed of movement of downhole pipe strings relative to one another.